1. Field of the Invention
The present invention relates generally to capacitive measuring systems and, more specifically, to systems having a sensor shielded against the detrimental effects of external electric fields and circuits for capacitively measuring the volume charge density of a sample of material.
2. Description of the Related Art
Capacitive measuring systems have been used to measure dissolved solids and impurities in fluids such as water and oil. A capacitive sensor is a device having two electrically conductive electrodes and a non-conductive body that insulates the fluid from the electrodes. The electrodes are typically tubular in shape and concentric with one another. When a sample is placed in the body, the device defines a capacitance in response to the dielectric constant of the sample. The dielectric constant varies in response to the ion concentration in the sample, which, in turn, is related to the solid impurities. The capacitance can be measured by connecting a suitable oscillator and measuring circuit to the plates. Comparing the measured capacitance to a known capacitance provides information relating to the electrical properties of the sample. For example, the dissolved solids in a sample of water can be determined by comparing the measured value to that which is produced in response to a known pure (e.g., double-distilled) sample of water.
Conventional capacitive sensors of the type described above are of low precision. They cannot, for example, consistently measure ion concentrations in water below a few parts per million. Practitioners in the art have discovered that measurements may vary over a wide range under seemingly identical test conditions. Furthermore, many of these devices are limited in their ability to handle flows of material at high pressures, and are subject to the build-up of static electricity from the flowing material, the discharge of which can interfere with accurate measurement. It would be desirable to provide a capacitive measuring system having a high-precision sensor. This and other problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.
The present invention includes a capacitive sensor and an electronic measurement system. In a first exemplary embodiment, the sensor includes a tubular outer conductor, a tubular inner conductor coaxial with the outer conductor, and an electrically insulated chamber between the inner and outer conductors. The material sample to be measured is placed in the chamber or forced to flow through the chamber. The chamber electrically isolates the sample from the conductors. When the sample is introduced into the chamber, it defines the dielectric of a capacitor. The plates of the capacitor are defined by the inner and outer conductors.
In a second exemplary embodiment, the sensor comprises one or more generally tubular channels or chambers through which the sample flows. Parallel conductor plates are located on each side of the channels, so that the sample flows between the conductors. Where two channels are used, three conductive plates are provided so that each channel is positioned between a pair of plates.
It has been discovered in accordance with the present invention that measurements produced by capacitive sensors known in the art are detrimentally affected by external electric fields, i.e., fields produced by environmental sources external to the sensor, such as fluorescent lights. A conductor exposed to an electric field acts as an antenna and develops a potential. If measurements are taken using such a sensor in, for example, a room having fluorescent lighting, the measurements will be markedly different than if taken in a room not having fluorescent lighting. Even in the absence of fluorescent lighting and other apparent sources of electric fields, the body of the person taking the measurements may emit sufficient electromagnetic radiation to affect the measurements.
To reduce the detrimental effect from electric fields, the present invention can include electrostatic shielding that completely encloses the chamber in which the sample is contained during measurement. In an exemplary embodiment, the sensor has one or more openings, and a removable cap made of a conductive material is attachable the opening. No external electric field can penetrate into the chamber because the cap is in electrical contact with the outer conductor and seals the opening during measurement. The electrostatic shielding can be incorporated into a housing, which can be a generally rectangular body formed from an insulating material that is coated with a conductive material. In another exemplary embodiment, the sensor has one or more openings, and a valve selectably opens or closes the opening. No external electric field can penetrate into the chamber because the valve, which may be solenoid-operated, has a conductive member that is in electrical contact with the outer conductor and seals the opening during measurement. In yet another exemplary embodiment, the sensor has one or more openings in which a screen made of a conductive material is disposed. An external electric field cannot penetrate into the chamber to any significant extent because the screen is in electrical contact with the outer conductor.
In addition to providing electrostatic shielding and general physical protection, the housing for the sensor can provide means for stabilizing the relative positions of the sensor components. In an exemplary embodiment, the housing can include a ribbed insert which supports and surrounds the tubular flow channels and the parallel plate conductors. The ribs of the insert extend along a plane orthogonal to the axes of the flow channels, resisting expansion of the channels under temperature or pressure stresses and maintaining a constant distance between the conductors and the flow channels. Additional stability can be provided by filling the ribbed insert with an epoxy material after attachment of the flow channel and conductor plate assembly. Once set, the epoxy provides significantly increased pressure tolerance, so that the sensor can be used in high pressure and/or high flow rate conditions. The epoxy can be selected to provide improved thermal insulation. The ribbed insert provides additional stabilization by minimizing shrinkage of the epoxy as itsets. In an alternative assembly procedure, the epoxy can be injected through a port in the housing after all components have been assembled, thus sealing all components to provide stability and improved protection against intrusion of dirt and moisture.
Another source of potential electrical noise that can detrimentally affect the measurement is static electricity generated by the rapid flow of material through the pipeline. This can particularly be a problem where the material contains conductive materials, such that the material has a low overall impedance. This static electricity can discharge in the sensors, resulting in spurious signals, inaccurate measurements, and can even damage the sensor""s electronics. The sensor can be fitted with one or more static discharge conductors, each comprising a conductive button or extension which extends into the flow channel. In the preferred embodiment, one button is positioned at each inlet and outlet of the sensor. A wire connected to each button is grounded at its second end, typically via a ground connection on the sensor""s circuit board, allowing the static to be harmlessly discharged. The button material should be inert and non-reactive relative to the material being measured since it will be in direct contact with the material. Appropriate materials can include coatings of metals such as gold, nickel or platinum.
The electronic measurement system determines the difference between a reference signal or a value representing such a signal having a constant reference frequency and a test signal or value representing such a signal having a test frequency responsive to the sensor capacitance. In an exemplary embodiment, the circuit includes D-type flip-flops that, in effect, subtract one signal from the other. In another exemplary embodiment, the circuit includes a phase-locked loop. The circuit may also include analog or digital means for improving response linearity, such as a logarithmic amplifier, look-up table, or polynomial correction. Also, the electronic measurement system may, in certain embodiments, periodically reverse the polarity of the test signal applied to the sensor in order to minimize charging of non-conductive surfaces, which promotes measurement accuracy and provides a cleaning effect and other benefits.
Temperature-compensation circuitry can be included to minimize drift in measurements arising from environmental and material temperature variations. The actual compensation is performed by the sensor microprocessor or other controller. The material temperature can be measured by conducting the temperature to a thermistor or other appropriate temperature measuring device. In an exemplary embodiment, a thermistor is integrated into the button used for static discharge, taking advantage of the thermal conductivity of the electrically-conductive coating of the button. The button is formed with a cavity in which the thermistor is sealed in place using a thermally-conductive epoxy or other thermally-conductive material. Above the thermally-conductive epoxy, between the button and the circuitry to which it attaches to the sensor""s circuit board, is a thermally-insulating epoxy or other seal which prevent the transfer of heat along any pathway but the thermistor""s wires. A single thermistor should provide the necessary data and can be included in either the inlet or outlet ports. However, multiple thermistors can be utilized, with one installed at each of any combination of ports if desired.
The system may be used for a variety of purposes, including measuring the extent of impurities in fluids, such as gases and water and other liquids, and measuring the flow rate of such fluids. Application of such a system can range from, for example, a purified water handling system in an integrated circuit manufacturing facility to desalination plants to oil or gas pipeline monitoring. In addition to the sensors, other components of the measurement system include communications interface modules and display devices, controllers and system software.
The foregoing, together with other features and advantages of the present invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.